I don't have the Pro. I used both my F8 and F8n as an interface, temporarily, for proof of concept with the first, and then the 2nd when I was between interfaces on a new Mac. I remember I wasn't happy about the lack of routing capabilities, but I didn't really investigate it - could have been laziness on my part! But, I don't recall it locking up. My personal assessment was that it was Ok "in a pinch" but that I couldn't really live with it as my recording interface. (I always loved these boxes for "field" recording though.)
First, I assume you've RTFM, of course. I didn't look to see how much info was in that, but that's the first place to go recheck. Then, Curtis Judd's YouTube channel may have some info on your model. (I watched a lot of his videos once upon a time.)
So, is your Mac overloaded with other background processes, or external devices that could be clogging things up? I can't quite understand why it would lock up on playback - you mean just playing a YouTube video, or what?
Again, no "Pro" so my experience is certainly not 100% aligned with your device and is a few years old, too... But, I'm looking at the menu on my F8n now, and wonder if you have it set for just the Audio Interface mode or Audio Interface with Rec one? Does your project sample rate and bit depth match the F8n's? Etc. You might need to check both the System Sound and the Audio MIDI Setup audio settings and get everything lined up with the F8n, too.
Also, not quite sure what "arm the F8n Pro's 'Mic In'" means - you just mean setting the track in Logic to record that is assigned to one of of the F8n Pro's inputs?